Various routing attacks for single-path routing have been identified for wireless ad hoc networks and the corresponding counter measures have been proposed in the literature. However, the effects of routing attacks on multi-path routing have not been addressed. In this paper, the performance of multi-path routing under wormhole attack is studied in detail. The results show that multi-path routing is vulnerable to wormhole attacks. A simple scheme based on statistical analysis (called SAM) is proposed to detect such attacks and to identify malicious nodes. Comparing to the previous approaches (for example, using packet leash), no special requirements (such as time synchronization or GPS) are needed in the proposed scheme. Simulation results demonstrate that SAM successfully detects wormhole attacks and locates the malicious nodes in networks with different topologies and with different node transmission range. Moreover, SAM may act as a module in local detection agents in an intrusion detection system (IDS) for wireless ad hoc networks.